PROJECT SUMMARY The normal growth, development, and reproduction of an organism relies on a defined set of essential chromosomes. At conception, the gain or loss of whole chromosomes is generally lethal and is thought to be the leading cause of spontaneous abortions in humans. Though the presence of a whole chromosome can be lethal, the gain of only part of a chromosome can be viable. Small supernumerary marker chromosomes (sSMCs) are structurally abnormal chromosomes that are present in ~1/2300 births and have been linked to infertility, fetal loss, and intellectual and developmental disabilities. Despite their clear impact on human health, very little is understood about important aspects of sSMC biology. Patient studies and human cell lines have been instrumental in determining the composition and frequency of existing sSMCs, but these systems are unable to model sSMC dynamics and formation during complex developmental processes. To close this knowledge gap, I will use the supernumerary B chromosomes recently found in D. melanogaster as a tractable model system to gain fundamental insight into supernumerary chromosome biology during female meiosis. Similar to some sSMCs described in humans, the B chromosomes can disrupt meiotic chromosome segregation, are subject to sex-specific differences in transmission frequency to progeny, and were formed from an essential chromosome. I will determine how B chromosomes disrupt the segregation of essential chromosomes during female meiosis using a genetic approach to monitor chromosome missegregation relative to a wide-ranging number of B chromosomes. I will then complement this genetic analysis with the visualization of live female meioses to gain a comprehensive view of meiotic supernumerary chromosome dynamics. To elucidate how B chromosomes are preferentially transmitted during female meiosis, I will identify differences in the centromeres of the B chromosomes and the essential chromosomes with high resolution by measuring the centromeric domains on chromatin fibers and conducting long-read sequencing and assembly of centromeric regions. Finally, I will determine the mechanism of supernumerary chromosome formation by strategically marking one of the essential chromosomes so that I can easily identify the formation of a new supernumerary chromosome. Based on the unique arrangement of markers this new supernumerary chromosome carries, I will deduce the mechanism by which it formed. Together, this work will allow me to gain research training in several areas that are essential for the continued investigation of supernumerary chromosome dynamics and formation, providing a foundation for my future research as an independent investigator examining how supernumerary chromosomes promote infertility, reduce gamete quality, and form de novo during meiosis.